This invention relates to fuses of a high current interruption capacity where the fuses blow to create high energy arcs at currents of the order of magnitude of at least about 1,000 amps. (Such a fuse excludes, for example, automobile fuses). These high current interruption capacity fuses commonly have a fuse element electrically connected between two cylindrical end cap terminals fitted over the ends of a sand-filled cylindrical envelope. However, some aspects of the invention are not limited to a fuse with a cylindrical envelope, and thus is applicable to one of the "prismatic" type.
In miniature and sub-miniature fuses of this type, operated at relatively low voltages (up to, for example, 150 V), a desired interrupting capacity of 1500A is not unusual and the interruption of higher current intensities of, for example, 5 to 6 kA is sometimes required. When such high currents are interrupted, high energy arcing occurs. If this arc is not quenched before it spreads to the metal end terminals of the fuse, the interruption is not effective and the fuse can undesirably explode. In order to quench the arc before this occurs, fuses designed for use with high interrupting capacity are commonly filled with sand. The manufacture of such sand-filled fuses, however, involves higher cost than the manufacture of non-filled fuses. There is accordingly a need for fuses which need not be filled with sand or the like but yet have a safe high current interrupting capacity.
To this end, various arc-quenching means other than sand have been developed. One such means are solid arc barriers in the form of compressible plugs or the like intimately surrounding the ends of the fuse element to act as barriers to the passage of the arc. Other means utilize plugs of material which substantially completely vaporizes under fuse blowing conditions to fill the fuse envelope with a gas which quenches the arc before it can reach the end terminals of the fuse.
Netherlands patent application No. 8005419 discloses a fuse whose cylindrical envelope is built up from a tubular outer member and a tubular inner member. The inner member is made of a material of high termal conductivity and low thermal shock resistance. When an electrical overload current is passed through the fuse element, the inner member is fragmented. The resulting heat and vapors are absorbed by the fragmented parts which may inhibit the build up of fuse-exploding arcs. This known fuse requires an envelope built up from two concentric tubes. This requirement increases the cost of production. The ability of this fuse to withstand safely high interruption currents is unknown.
One object of the present invention is to provide a fuse of the high current interrupting type which avoids the build-up of fuse-exploding arcs by unique and inexpensive means.